SFP stands for small form-factor pluggable. They function the same as a GBIC however, they are much smaller in size.
In networking, a GBIC is used to interface a fiber optic system with an Ethernet system, such as Fibre Channel and Gigabit Ethernet.
SFP+ is a Small Form Factor Pluggable 10Gbps transceiver. Functions the same as XENPAK, X2, and XFP but in a smaller package
XENPAK is a standard that defines a type of fiber-optic or copper transceiver module which is compatible with the 10 Gigabit Ethernet (10GE) standard.
XFP is a small form factor pluggable 10Gbps transceiver. Functions the same as XENPAK and X2, but comes in a much smaller package.